The present invention relates generally to load control systems, and more particularly, to a lighting control system having an overload protection circuit to limit the power dissipation of a switching element in the control system from exceeding a predetermined maximum level.
Phase-controlled lighting controllers are well known and perform dimming functions by selectively connecting an AC power source to a load during each half-cycle. The AC power may be switched using controllably conductive devices such as triacs, anti-parallel SCRs, field effect transistors (FETs) or insulated gate bipolar transistors (IGBT). The amount of dimming is determined by the ratio of xe2x80x9cONxe2x80x9d time to xe2x80x9cOFFxe2x80x9d time of the controllably conductive device. In conventional forward phase-controlled dimming, the controllably conductive device (triac or SCR) is OFF at the beginning of each half-cycle (i.e., at the zero crossing) and turned ON later in the half-cycle. In reverse phase-controlled dimming, the controllably conductive device (FET or IGBT) is switched ON to supply power to the load at or near the zero crossing and is switched OFF later during the half-cycle. For each method of phase-controlled dimming, the ratio of ON time to OFF time is determined based on a user-selected desired intensity level.
Lighting controllers are rated to control a predetermined maximum load. If the controller is overloaded the maximum temperature rating of the controllably conductive device may be exceeded and the device will not last as long as a properly loaded device or fail catastrophically rendering the controller useless. A lighting controller can easily be overloaded by an installer who connects too many lamps to the controller or by a maintenance person who replaces failed lamps with higher wattage lamps.
Another factor that may lead to an elevated device temperature is operating the lighting controller in an elevated ambient temperature. Lighting controllers are rated to operate in an ambient temperature range usually 0 to 40xc2x0 C. An elevated ambient temperature would cause an otherwise properly loaded device to operate above its safe operating temperature.
Several methods of sensing overload conditions may be found in the prior art. For example, U.S. Pat. No. 5,325,258, to Choi et al., discloses a gate driver circuit that uses sense resistors to determine the current flowing through a low side and high side FET. While the FET is being driven (i.e., ON), a voltage across the sense resistor is compared to a fixed threshold voltage. If the voltage across the sense resistor remains above the fixed threshold for a period of time set by a blanking circuit, the FET is determined to be overloaded and shut down. The blanking circuit is provided to prevent spurious signals from shutting down the FET driver. While Choi et al. prevents overload conditions under certain circumstances, it would fail to detect a short circuit condition during the blanking period. Also, because Choi et al. compares the current passing through the FET to a fixed threshold, the device may not accurately detect overcurrent conditions that occur early in the ON period of each half cycle.
U.S. Pat. No. 5,010,293, to Ellersick, discloses a current limiting circuit for a power FET. A bipolar transistor is connected to shunt the gate of the power FET to the potential at its source when the bipolar transistor is conducting in order to limit the current passing through the power FET. A sense resistor is provided in series with a conductor path for controlling a base element of the bipolar transistor to cause the transistor to conduct when current through the sense resistor exceeds a predetermined amount. However, the Ellersick circuit is limited because it compares the current passing through the FET to a fixed threshold, which may not accurately detect overcurrent conditions early in the ON period of each half cycle and because the power FET becomes active to limit the current which dissipates a lot of power.
U.S. Pat. No. 5,079,456, to Kotowski et al., discloses a current monitoring circuit that includes a smaller sense FET that carries a current proportional to a larger power FET in the device. A comparator senses the voltage across the smaller transistor to indicate if the current in the sense transistor exceeds a predetermined amount equal to a maximum source current of the sense transistor. A second embodiment regulates the source current through the sense transistor in order to regulate the current through the power transistor wherein the sense transistor is operating in the linear region. By modifying the drain to source voltage of the sense transistor the device can regulate the current carried by the power transistor. A particular disadvantage of the Kotowski et al. system is that it requires a separate sense FET to monitor the power FET, which adds to the complexity and cost of the monitoring circuit. Again, the FET becomes active to limit the current which dissipates a lot of power.
U.S. Pat. No. 4,937,697, to Edwards et al., discloses another protection circuit that monitors instantaneous FET drain to source voltage to provide a current sense signal. When the current sense signal exceeds a predetermined reference limit signal, a first control circuit turns the FET OFF instantly. A reference generator provides a reference limit signal having a predetermined temperature variation as a function of the sensed temperature of the FET such that current limits may be set for low device temperatures. A second control circuit is provided to protect against overcurrent conditions created by short circuits by turning the FET OFF when sensed FET current exceeds a predetermined limit after a delay. The delay circuit inhibits operation of the control circuits until a predetermined time after the FET is turned ON. During this time there is no protection.
While each of the systems described above attempts to prevent overloading and overheating of the controllably conductive devices for their particular applications, they require the use of more costly hardware or fail to provide adequate protection over a wide range of operating conditions and environments. In addition, the devices of the prior art function to limit the flow of current through the controllably conductive device in overload conditions by modifying the drain to source voltage, which does not reduce the overall power dissipation in the FET. The load control circuit of the present invention reduces the current flow to a safe operating level while not increasing dissipation in the FET. The present invention provides a solution to these problems.
In accordance with a first aspect of the present invention, there is provided a protection circuit for use in a load control system for limiting power dissipated by an electronic component that switches an AC source to a load. The electronic component may be, e.g., a field effect transistor. The protection circuit includes an integrating circuit which integrates a measured parameter of the electronic component over a predetermined period of time and produces an output value, a threshold generating circuit which generates a first threshold indicative of a maximum average power dissipation of the electronic component, and a comparator circuit which compares the first threshold and the output value. The comparator provides a signal to turn OFF the electronic component when the output value exceeds the first threshold.
In accordance with a feature of the invention, the first threshold may be determined in accordance with an ON-state resistance of the electronic component and the measured parameter. Further, the first threshold may have a variable value that changes during one-half of a period of a fundamental frequency of the AC source. The predetermined period of time may begin when the AC source crosses a zero potential, and have a length no longer than one-half of a period of a fundamental frequency of the AC source.
According to other features of the invention, the protection circuit may include a reset circuit that holds OFF the integrating circuit during a period of time that the electronic component is normally OFF. A filtering circuit may be provided that receives the signal from the comparator circuit to smooth the control of the electronic component in accordance with a time constant of the filtering circuit. The protection circuit may further include an error generating circuit that receives an output of the filtering circuit and compares the output of the filtering circuit to a second threshold. The error generating circuit may turn OFF the electronic component based on the second threshold. The second threshold may vary in accordance with an ON-state resistance of the electronic component and the maximum average power dissipation of the electronic component. Further, the second threshold may be identical to the first threshold.
In accordance with another aspect of the present invention, there is provided a load control system for delivering power from an AC source to a load. The load control system includes a zero cross detector that monitors the AC source having a fundamental frequency, at least one switching element that selectively connects the AC source to the load, a sensing circuit that senses an instantaneous ON-state parameter of the at least one switching element and produces an output, an overload circuit that determines if the at least one switching element is in an overload condition, a short circuit protection circuit that also receives the output to determine if the at least one switching element is shorted, and a controller that controls the load control system.
The controller of the load control system receives information from the zero cross detector and outputs a gate drive signal to turn the at least one switching element ON. Also, the overload circuit receives the output and determines an integrated value of the ON-state parameter and compares the integrated value to a threshold indicative of a maximum average power dissipation of the at least one switching element to make its determination if the switching element is in an overloaded state, and reduces the ON time of the at least one switching element when the at least one switching element is determined to be overloaded. Further, the overload protection circuit may include the features of the above-noted protection circuit. The short circuit protection circuit also reduces the ON time of the at least one switching element when the at least one switching element is determined to be shorted. The OFF-state voltage of the at least one switching element may not be detected to improve accuracy of the overload circuit.
The load control system may be used to control capacitive loads, and in particular may be used to control a lighting load. In such an environment, the controller sets an ON-time of the at least one switching element to a constant duty cycle for a given intensity level of the lighting load set by a user. Further, the overload conditions may be visually indicated to a user by flashing the lighting load.
According to features of the invention, the load control system may also include a power supply that is connected to the AC source and outputs a regulated voltage to the controller. A gate drive circuit may be included that receives an output of the overload circuit and the short circuit protection circuit to turn OFF the at least one switching element. The gate drive circuit turns OFF the at least one switching element based on a predetermined prioritization, wherein the short circuit protection circuit has priority over the overload circuit, and the overload circuit has priority over the controller to turn OFF the at least one switching element.
In accordance with yet another aspect of the present invention, there is provided a method of protecting a switching element connected between an AC source and a load from dissipating power in excess of a predetermined amount. The method comprising measuring a parameter of the switching element; integrating the measured parameter over a predetermined time period to produce an output; comparing the output to a variable threshold; producing a signal when the output exceeds the variable threshold; and turning OFF the switching element in response to the signal. The switching element may comprise a field effect transistor (FET), and the ON-state parameter may be a selected one of a voltage across the FET, a current through the FET, or a temperature of the FET.
According to features of the invention, the switching element may be turned OFF when the instantaneous ON-state parameter exceeds a second threshold value. Further, a visual indication may be provided to a user that the switching element has been overloaded by, e.g., cycling power to the load by turning OFF and ON the switching element.
Additional aspects and features of the present invention are detailed below.